Semiconductor dice are normally packaged in a plastic capsule to protect them from physical damage. Connections are made to external circuitry, such as a printed circuit board (PCB), by means of metal pieces, called “lead fingers,” that protrude from the capsule and can be soldered or otherwise connected electrically. Alternatively, in a “no lead” package the metal pieces have faces that are flush with the surfaces of the capsule.
FIGS. 1A-1D show four configurations of a semiconductor package. Each package contains a semiconductor die 10, a die-attach pad or “paddle” 12, metal pieces or lead fingers 14, and a plastic capsule 16. Die 10 is attached to attach pad 12 by solder or epoxy. Bonding wires 18, which may be made of gold or aluminum, for example, connect terminal pads on die 10 to metal pieces or lead fingers 14 and to die-attach pad 12. Typically, these connections, often referred to as “wire bonds,” are made by thermosonic or ultrasonic welding. Die-attach pad 12 and metal pieces or lead fingers 14 were traditionally made of copper alloy coated with silver or nickel/palladium/gold. The terminal pads on die 10 are traditionally made of aluminum. However, for low-voltage, high frequency semiconductor chips copper, which has a lower resistivity than aluminum, has become the material of choice. After the electrical connections have been made, die 10, die-attach pad 12 and the inner portions of lead fingers 14 are placed in a mold and a molten plastic compound is introduced into the mold to form capsule 16.
FIG. 1A shows a “no lead” package 1 wherein the edges of metal pieces 14 are flush with the side and bottom surfaces of capsule 16. FIG. 1B shows a “gull wing” package 2 wherein lead fingers 14 extend from the opposite sides of capsule 16 and are bent or curved such that their ends are substantially parallel to the bottom of capsule 16. FIG. 1C shows a “dual in-line” package 3 wherein lead fingers 14 extend from the opposite sides of capsule 16 and are bent vertically downward in a directional perpendicular to the bottom of capsule 16. FIG. 1D shows a “J” or “C” lead package 4 that is similar to the dual in-line package 3 except that the ends of lead fingers 14 are bent inward as shown. In an alternative set of packages 5-8, shown in FIGS. 2A-2D, respectively, die-attach pad 12 is exposed at the bottom instead of being completely encased in plastic capsule 16. This can be done, for example, to provide better heat transfer from die 10.
It is highly desirable to make the package as rugged as possible, in particular to ensure that it is able withstand extremes of temperature and humidity and numerous cycles between high and low temperatures. The plastic that is used to manufacture the capsule normally has a thermal-expansion coefficient that is quite different from the thermal-expansion coefficient of the semiconductor chip, leadframe and die-attach material. As a result, as the package cycles between high and low temperatures, stresses are created at the interfaces between the metal and plastic. Over time, this can lead to fractured wire bonds. Moreover, the stress at the metal-plastic interfaces can allow moisture to enter the package. This moisture can corrode the metal components of the package and semiconductor chip. If the package is subjected to freezing temperatures, the moisture can expand and cause separation between the plastic molding compound and the metal. Conversely, if the package is exposed to very high temperatures, the moisture can turn to steam and likewise cause separation or, in extreme cases, cause the package to experience a “popcorn” effect.
Since the lead fingers are often soldered to external circuitry (e.g., a PCB) it is predictable that in many cases the package will be exposed to extremely high temperatures as a result of the soldering process. This situation as worsened recently as a result of the restrictions on the use of lead in the solder that is used to make the connections between the lead fingers of the semiconductor package and the PCB. With solder that contains 15% lead, for example, the lead fingers must be heated to a temperature of approximately 240° C. With a lead-free solder, this temperature increases to approximately 260° C.
Accordingly, there is a real need for a semiconductor package, particularly one that contains a copper alloy leadframe, that is highly impervious to the ingress of moisture.